1. Field of the Invention
This invention relates to a power supply, and particularly to a power factor correction power supply.
2. Description of the Related Art
Conventionally, two-stage converters are widely used to limit harmonics of line currents of electronic equipments to certain levels. A first stage is a power factor correction (PFC) stage comprising an inductor, a rectifier and a transistor, and the transistor is driven by a PFC signal from the PFC stage. A second stage comprises a transistor controlled by a PWM signal, a transformer and a secondary circuitry. By way of the two-stage structure, an output voltage is regulated and output noise is reduced.
However, the PFC stage increases the cost and the number of devices of the power supply, and therefore the efficiency of the power supply is reduced. Other approaches in the art use single stage converters integrating functions of power factor correction and isolated DC/DC conversion into a single stage. Unfortunately, a common problem with these approaches is an inherent low efficiency due to a fact that energy is processed twice during an energy transferring process, which causes an extremely high switching loss.